s390x: fix generation of event information crw

Only one channel report word (crw) may be pending if there is
event-information pending.

This patch introduces a bool-type field 'sei_pending' for the
channel subsystem, which indicates whether there are pending events.
It is set when event information is made pending and the crw
generated, and cleared after the guest has collected all pending
event information. A crw is not generated if this flag had already
been set.

Signed-off-by: Song Shan Gong <gongss@linux.vnet.ibm.com>
Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>